Migrate the UserService/Model to not be a singleton

- Rename userService to userModel on appContext
- Merge user-model.ts and user-service.ts
- Rename UserService to UserModel
- Move all observables onto UserModel
- Inject UserModel in the models/services that were directly accessing
the observables

Google-Bug-Id: b/206459178, b/207628953
Change-Id: Icf4f81c877efb58e289678a7331bde7254142a58
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
index 0d63360..fd30c6a 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts
@@ -50,7 +50,7 @@
   // Private but accessed by tests.
   readonly browserModel = getAppContext().browserModel;
 
-  private readonly userService = getAppContext().userService;
+  private readonly userModel = getAppContext().userModel;
 
   private subscriptions: Subscription[] = [];
 
@@ -83,7 +83,7 @@
    */
   setMode(newMode: DiffViewMode) {
     if (this.saveOnChange && this.mode && this.mode !== newMode) {
-      this.userService.updatePreferences({diff_view: newMode});
+      this.userModel.updatePreferences({diff_view: newMode});
     }
     this.mode = newMode;
     let announcement;